FIG. 1 is a block diagram of a network structure of E-UMTS (evolved universal mobile telecommunications system) to which a related art and the present invention are applicable. An E-UMTS is the system evolving from the conventional UMTS and its basic standardization is currently handled by the 3GPP.
Referring to FIG. 1, an E-UMTS network includes a user equipment (hereinafter abbreviated ‘UE’), a base station (hereinafter named ‘eNode B’) and an access gateway (hereinafter abbreviated ‘AG’) connected to an external network by being located at an end of the E-UMTS network. And, at least one cell can exist in one eNode B.
Layers of a radio interface protocol between UEs and a network can be classified into a first layer L1, a second layer L2 and a third layer L3 based on three lower layers of OSI (open system interconnection) reference model widely known in communication systems. A physical layer belonging to the first layer L1 offers an information transfer service using a physical channel. And, a radio resource control (hereinafter abbreviated ‘RRC’) located at the third layer plays a role in controlling radio resources between the UE and the network. For this, the RRC layer enables RRC messages to be exchanged between the UE and the network. And, the RRC layer can be distributively located at network nodes including an eNode B, an AG and the like or at either the Node B or the AG.
FIG. 2 is an architectural diagram of a control plane of a radio interface protocol between a UE (user equipment) and a UTRAN (UMTS terrestrial radio access network) based on the 3GPP radio access network standard. Referring to FIG. 2, a radio interface protocol vertically includes a physical layer, a data link layer, and a network layer and horizontally includes a user plane for data information transfer and a control plane for signaling transfer. The protocol layers in FIG. 2 can be divided into L1 (first layer), L2 (second layer), and L3 (third layer) based on three lower layers of the open system interconnection (OSI) standard model widely known in the communications systems.
The respective layers of a radio protocol control plane shown in FIG. 2 and a radio protocol user plane shown in FIG. 3 are explained as follows.
First of all, the physical layer as the first layer offers an information transfer service to an upper layer using a physical channel. The physical layer PHY is connected to a medium access control (hereinafter abbreviated ‘MAC’) layer above the physical layer via transport channels. And, data are transferred between the medium access control layer and the physical layer via the transport channels. Moreover, data are transferred between different physical layers, and more particularly, between one physical layer of a transmitting side and the other physical layer of a receiving side via the physical channel.
The medium access control (hereinafter abbreviated ‘MAC’) layer of the second layer offers a service to a radio link control layer above the MAC layer via a logical channel.
The radio link control (hereinafter abbreviated RLC) layer of the second layer supports reliable data transfer. A PDCP layer of the second layer performs a header compression function for reducing unnecessary control information to efficiently transmit data, which is transmitted using such an IP packet as IPv4 or IPv6, in a radio section having a relatively small bandwidth.
A radio resource control (hereinafter abbreviated ‘RRC’) layer located on a lowest part of the third layer is defined in the control plane only and is associated with configuration, reconfiguration and release of radio bearers (hereinafter abbreviated ‘RBs’) to be in charge of controlling the logical, transport and physical channels. In this case, the RB means a service offered by the second layer for the data transfer between the UE and the UTRAN.
As a downlink transport channel carrying data to UEs from the network, there is a BCH (broadcast channel) carrying system information and a downlink SCH (shared channel) carrying user traffic or control messages. The traffic or control messages of a downlink multicast or broadcast service can be transmitted via the downlink SCH or a separate downlink MCH (multicast channel). Meanwhile, as an uplink transport channel carrying data to the network from UEs, there is a RACH (random access channel) carrying an initial control message and a UL-SCH (uplink shared channel) carrying user traffic or control message.
In the related art, a network is unable to know whether a UE is able to perform a prescribed measurement. For instance, if a UE receives a multimedia broadcast/multicast service (hereinafter abbreviated MBMS) via a point-to-point radio bearer or a common control message, a network is unable to know whether the UE receives the MBMS or common control messages. So, the network is able to command the UE to perform an inter-frequency measurement while the UE is receiving the MBMS or the common control messages. However, in this case, the UE may be unable to receive a specific service while performing the inter-frequency measurement.